JP2019506701A - Anode with diatom shell - Google Patents

Abstract

【Task】
A composite comprising a porous silicon dioxide network coated with a carbon coating, a conductive filler such as carbon black, and a water dispersible or water soluble binder, preferably an alginic acid binder.
[Selected figure] Figure 1

Description

  The present invention relates to lithium ion batteries and more particularly to an anode for such batteries. The present invention relates to a composition of materials suitable for lithium ion battery anodes, said composition comprising a carbon coated calcined diatom, a conductive filler (eg carbon black, carbon nanotubes etc) and a water soluble Containing a water / water dispersible binder. The invention also relates to a process for the preparation of the carbon coated diatoms used in the invention and to a battery comprising the anode of the invention.

  Lithium ion batteries are often used in household appliances. Lithium ion batteries (sometimes referred to as Li ion batteries or LIB) are a type of rechargeable battery type in which lithium ions move from the negative electrode to the positive electrode during discharge and return to the negative electrode during charge. There are various known electrode materials used in Li-ion batteries. Li-ion batteries, for example, often use lithium intercalation compounds as one electrode material. An electrolyte that allows ion migration and the two electrodes described above are components of a lithium ion battery cell.

  Properties such as chemical properties, performance, cost and safety vary with LIB type. Efforts have been expended in recent years to research cathodes in an attempt to develop high performance cathodes to improve cell performance, but no results have been obtained. Thus, to obtain high performance Li-ion cells, developing high performance anodes is a more realistic option to improve the overall cell specific capacity.

Handheld electronics most often use anode based LIBs containing graphite. Graphite is preferred over other potential anode materials, such as lithium metal, because it has a limited theoretical capacity but is a safe material. During the charge cycle, the insertion of Li ions occurs in the graphite to form a LiC _6. This process involves a slight expansion of only about 12% by volume. The theoretical capacity for the cycle between C and LiC ₆ is 372 mAh g ⁻¹ .

As a possible alternative to graphite anodes, many metals (ie Sn, Pb, Al, Ge, Zn, Cd, Ag and Mg) which are alloyed with lithium and which have high specific capacities have been studied in the last decade ing. However, most of these alloys are inferior in cyclability because they undergo extensive expansion during cycling resulting in complete decomposition and loss of electronic contact with the current collector. The exception is a TiO _2. Its theoretical capacity is 335 mAh g ^-1 and its thermal stability is good. However, this lithium titanium negative electrode material operates at relatively high voltages (> 1 V vs. Li / Li +), which leads to a reduction in cell voltage and hence a corresponding reduction in energy and power density. Thus, the interest for lithium titanate negative electrodes has diminished, except where rapid charging is crucial.

More recently, silicon anodes have attracted attention. Silicon is an ideal anode material because it has a very high theoretical capacity. The Si-based anode has the highest theoretical specific capacity (4200 mAh g ⁻¹ ) among the elements forming the alloy. The silicon-based anode can easily reach a target specific capacity value of 1000-1200 mAh g ⁻¹ . However, the rapid volumetric expansion of the Si anode during cycling results in capacity fade and micronization, requiring a high degree of processing to make it a viable electrode in Li-ion batteries.

  The Si anode has an expansion of 300% by volume and is unsuitable for industrial use. With a 300 volume percent expanded anode, the material is subjected to very high mechanical stress, resulting in volume loss of the anode and subsequent micronization, as well as degradation of the solid electrolyte interphase that separates the current capture portion of the anode from the electrolyte.

  Attempts have been made to address the problem of inflation. U.S. Patent Application Publication No. 2008/0038170 proposes a shaped Si nanoscale material obtained from diatoms after appropriate processing. Similarly, in Chinese Patent No. 102208636 (patent document 2), diatomaceous earth is taken and metallogenic reduction of diatomaceous earth to Si is carried out before coating with carbon. The idea is to use a porous diatomaceous earth structure with the advantages of Si as an electrode, but this solution still suffers from the expansion problems described above.

  Thus, we are examining the use of diatoms as an alternative anode to existing graphite based anodes that provide better electrochemical performance in terms of capacity, cyclability and stability.

High theoretical reversible specific capacities (about 1675 to 1965 mAh g ^-1 ) and low discharge potential silica (SiO ₂ ) have recently attracted great interest as potential substitutes for Si-based anodes. Since the negative voltage of the silica is higher than that of the graphite-based anode, the energy density decreases due to the low cell voltage. However, this reduction in energy can be compensated by its very high reversible specific capacity. The combination of high voltage cathode and composite silica anode can increase the energy density of Li-ion batteries to the requirements of current and future applications.

  Diatoms are unicellular algae that produce a nanostructured amorphous silica framework called the putamen. In Energy Environ Sci 2011, 4, 3930 (Non-patent Document 1), diatoms are said to have potential in nanobiotechnology because they can support metal oxides. These materials have also been proposed for use in batteries, for example by conversion of putamen to Si metal. So we noticed that diatoms could be used for the high performance anode.

Diatoms are algae and a natural source of silica (SiO ₂ ). These algae grow the cell wall of silica called putamen. These mantles provide a natural, ordered nanostructured porous silica framework, which can compensate for the expansion that occurs during battery cycling. The organic matter contained in diatoms can also be used as a source of carbon coatings, which can reduce considerable processing time and costs associated with other coating processes. The use of silicon dioxide for the anode of Li-ion batteries has been attempted in the past. In J Power Sources 196 (2011) 10240-10243 (Non-patent Document 2), carbon-coated silicon dioxide nanoparticles are proposed as the anode of a Li-ion battery. The nanoparticles used are Aerosil (AEROSIL) particles, which are heated in sucrose solution to prepare silicon dioxide embedded in a carbon matrix. However, the material is not porous.

  U.S. Patent Application Publication No. 20140134503 (Patent Document 3) describes a manganese-containing putt on an electrode. Mn functions as a coating on the putamen. The putamen appears to be used merely as a mechanical support.

  In RSC Adv. 2014, 4, 40439 (Non-patent document 3), diatoms covered with red algae are pyrolyzed to form an anode for Li-ion batteries and used with a PVDF / NMP binder. However, the biomass is not purified before use and the binders used are non-aqueous / non-water soluble. However, the combination of calcined diatoms and water soluble binders is new and provides an environmentally friendly solution that is not only advantageous in terms of electrode capacity but also reduced in cost.

  In this connection, another major problem with graphite-based Li-ion battery solutions is that the binder used in these batteries is PVDF (polyvinylidene fluoride) in combination with NMP (N-methyl-2-pyrrolidone) It is a certain thing. PVDF is not an environmentally friendly substance. Because NMP is volatile, explosive, flammable, and toxic to reproduction, it is less safe. Batteries are commonplace in "green" products, such as electric vehicles, and there is a need in the industry for alternative "green" binders. In particular, there is a need in the industry for aqueous binders or water soluble binders. Such binders are not only environmentally incomparable, but also more than 90% cheaper than current solutions.

  In a detailed study on lithium ion batteries, switching from NMP soluble PVDF binder to aqueous binder reduces cost by over 90% in combination of 1) cheaper binder and solvent materials and 2) cheaper electrode processing process Was calculated to be possible.

  It has surprisingly been found that the aqueous binder is compatible with the calcined carbon-coated diatoms of the present invention. More particularly, algal based aqueous binders can be used to make the electrodes of the present invention. They are compatible with calcined diatomaceous material and improve the stability of the formed electrode during lithiation and delithiation.

U.S. Patent Application Publication No. 2008/0038170 Chinese Patent No. 102208636 U.S. Patent Application Publication No. 20140134503 U.S. Patent Application Publication No. 2014/0193712

Energy Environ Sci 2011, 4, 3930 J Power Sources 196 (2011) 10240-10243 RSC Adv. 2014, 4, 40439

  The water soluble / dispersible binder also interacts slightly with the electrolyte which helps to build a stable solid electrolyte interface (SEI). US Patent Application Publication No. 2014/0193712 describes the use of alginic acid binders in silicon based anodes, but not their use with porous calcined silica networks.

  Thus, the present invention in one aspect a composite comprising a porous silicon dioxide network coated with a carbon coating, a conductive filler such as carbon black, and a water dispersible or water soluble binder, preferably an alginic acid binder I will provide a.

  In another aspect, the invention provides a composite comprising a carbon coating coated calcined diatomaceous silica network, a conductive filler such as carbon black, and a water dispersible or water soluble binder, preferably an alginic acid binder. provide.

  The invention provides, in another aspect, a porous silicon dioxide network coated with a carbon coating, wherein the porous silicon dioxide network coated with the carbon coating comprises less than 1% by weight in total of KCl and NaCl. Provided is a porous silicon dioxide network.

  The invention, in another aspect, provides an anode for a lithium ion battery comprising a composite as hereinbefore described.

  The invention comprises, in another aspect, a lithium ion battery having at least an anode, a cathode and an electrolyte, said anode comprising a composite as hereinbefore described.

The invention, in another aspect, comprises
Calcining a diatom source in the presence of a carbon source to obtain a carbon-coated calcined diatom network;
Providing a method of making a composite as described herein comprising mixing the network with a conductive filler such as carbon black and a water soluble binder.

  Preferably, the mixture is in the form of a slurry, which is cast on a support such as battery grade Cu to form a film thereon.

  The invention provides in another aspect the product of the above process.

FIG. 1 shows a flow chart of the preparation of the composition of the invention. FIG. 2 shows the X-ray diffraction pattern of diatoms. (A) Baking at 600 ° C. in argon atmosphere without purification step, (b) Baking at 1000 ° C. in argon atmosphere without purification step, (c) Baking at 650 ° C. in air after washing step, and (d) After the washing step, bake at 600 ° C. in argon. FIG. 3 shows a TG curve of carbon-coated diatom (SiO ₂ -C). FIG. 4 shows the relationship between the residual carbon content of diatoms after calcination and the added starch content. FIG. 5 shows FESEM images of carbon coated diatoms (SiO ₂ -C). FIG. 6 shows the pore size distribution and pore surface area of carbon-coated diatom complex (SiO ₂ -C) measured by mercury intrusion porosimetry (MIP). FIG. 7 shows the fines of washed diatoms after firing in air at 600 ° C. for 2 hours and washing, and after firing carbon-coated diatoms (SiO ₂ -C) calcined in argon for 2 hours at 600 ° C. 7 shows BET surface area including pores / outer surface. FIG. 8 shows constant current charge / discharge profiles of carbon-coated diatom-based anodes (SiO ₂ -C) in the first, second, twenty-fifth and fifty-fifth cycles. FIG. 9 shows the percent irreversible capacity and specific capacity at current densities of 50 mA / g and 100 mA / g as a function of cycle number for carbon-coated diatom-based anodes (SiO ₂ -C). FIG. 10 shows the differential capacity plots of the carbon-coated diatom-based anode (SiO ₂ -C) at cycles 1, 25 and 50. FIG. 11 shows a comparison of the specific capacities of a commercial graphite anode and a carbon-coated diatom-based anode (SiO ₂ -C) at moderately high current densities (100 mA / g) using various potential windows. FIG. 12 shows the constant current charge / discharge specific capacity of carbon coated diatom based anodes (SiO ₂ -C) at different current densities (100, 200, 500, 1000 mA / g). FIG. 13 shows a carbon-coated diatom base based on carbon coatings obtained from different amounts of starch sources (0, 35 and 80 wt%) at low current densities (20 mA / g) using voltage windows (0 to 2.5 V) A comparison of the specific capacity of the anode of FIG. 14 shows the total specific capacity of Li-ion batteries as a function of anode specific capacity (C _A ). Consider cathodes with specific capacities of 100, 150 and 200 mAh / g. FIG. 15 shows a comparison of the total specific capacity of Li-ion batteries based on Equation 1 with diatom-based anode (SiO ₂ -C) and graphite anode, (a) theoretical specific capacity of graphite (372 mAh / g And (b) using the experimentally measured specific capacity of graphite (80 mAh / g) at a current density of 100 mA / g. Cases (FIGS. 15a and FIG. 15b) is also the specific capacity of the current density experimentally measured diatom based anode at _{100mA / g (SiO 2 -C)} (850mAh / g) is used. FIG. 16 shows (a) constant current charge / discharge specific capacity of a graphene-based anode at different current densities (100, 200, 500, 1000 mA / g) and (b) graphene (at 100 mA / g current density) 225 mAh / g) and a comparison of total specific capacity of Li-ion batteries based on equation 1 using the experimentally measured ratio volume of diatom based anode _{(SiO 2 -C) (850mAh /} g). Figure 17a illustrates the long cycle of the half cell (silica anode to Li counter electrode). FIG. 17 b illustrates the long cycle of the half cell (silica anode to Li counter electrode). FIG. 18 shows half cells cycled every other cycle at high and low discharge rates. In FIG. 19, the electrode is a diatom similar to that described above, coated with carbon and mixed with carbon black and Na-alginate using water as the solvent. FIG. 20 shows charge / discharge data for two half-cells, one cycled using CNTs as the conductive additive in the binder and one cycled using a Na-alginic acid binder without CNTs. A Nyquist diagram of both cells is shown in FIG. FIG. 22 shows the results of the all-cell test.

[Definition]
The present invention relies on a porous silicon dioxide network coated with a carbon coating. This network is obtained by calcination of a diatom source in the presence of a carbon source. Thus, as used herein, the terms porous silicon dioxide network and calcined diatoms refer to similar structures. This network is a porous carbon-coated silicon dioxide structure, which is obtained by calcining diatoms in the presence of a carbon source at a temperature of 400-800 ° C. under an inert atmosphere.

  The term water-soluble binder used is such that the binder used to combine the conductive filler, such as carbon black, with the porous silicon dioxide network coated with the carbon coating is water soluble, and the above-mentioned composite The production method shows that use in an aqueous solution is possible. The term water soluble means a substance having water solubility at 23 ° C. of at least 1 g / L, for example at least 10 g / L.

  Also, the binder may be considered water dispersible. Dispersibility means that the binder can be used in an aqueous environment (e.g. suspension, dispersion) in order to combine a conductive filler such as carbon black with a porous silicon dioxide network coated with a carbon coating. means.

  The binder also ensures adhesion between the porous silicon dioxide network, the filler and the current collector (eg Cu). However, of course, a drying step is used to form the final anode. Thus, during the manufacture of the anode, for example the water present in the solution of the binder can be removed during this drying process.

  Cleaned or washed, carbon-coated porous silicon dioxide networks have been subjected to a purification treatment, for example as described herein, to remove the salts present in natural marine diatoms, in particular NaCl and KCl. is there. The clean, carbon-coated porous silicon dioxide network is preferably free of NaCl and KCl. By "not included" is meant that the characteristic peaks associated with these compounds are not seen in the XRD obtained as shown in FIG.

<Porous silicon dioxide network coated with carbon coating>
The present invention relies on a porous silicon dioxide network coated with a carbon coating. Silicon dioxide networks are obtained by calcination of diatoms as described in more detail below in the presence of a carbon source. The term diatom, as used, refers to a type of algae which has a cell wall consisting of silicon dioxide. These cell walls are called putamen. There are many species of diatoms, all of which are suitable for use in the present invention. Diatoms are one of the most common types of phytoplankton. The diatoms are single cells, but form colonies in the form of filaments or ribbons (eg, Fragilaria etc.), fans (eg, Meridion etc.), zigzags (eg, Taberaria etc.), or stars (eg, Astelonela etc.) Can. A unique feature of diatom cells is that they are enclosed within the cell wall consisting of silica (hydrous silicon dioxide) called putamen. These putamen have various forms but are usually symmetrical and group names come from there.

  More than 200 genera are inhabited and approximately 100,000 species are estimated to exist. Diatoms are a widely distributed group found on the ocean, freshwater, soil, and wetland surfaces. Most of them float and inhabit open waters, but some also exist as surface coverings and even under moist atmospheric conditions. It is usually fine, but depending on the diatom species, it can reach up to 2 millimeters in length.

  Diatoms belong to a large group called heterophyllous algae, including both autotrophic organisms (eg, golden algae, kelp) and heterotrophic organisms (eg, aquatic fungi). Their yellowish-brown chloroplasts are unique to the indifferent algae, have four membranes, and contain pigments such as carotenoid-fucoxanthin.

  The diatoms transported from the northern Norwegian Sea are disk-shaped and have been identified as Coschinodiscus coskinodiscus.

  It will be appreciated that the diatoms used in the present invention may be mixed with different species. The diatoms are harvested as such, and the composition of the diatoms depends on the sea from which the diatoms are harvested.

  The diatom sheaths have a hierarchical nano or macroporous structure, which results in a porous structure in the calcined diatoms. As shown in FIG. 5, the nature of the putamen pore differs depending on the species. However, it is preferred that there be pores of size less than 90 nm, for example 50-80 nm in size. It is also preferred that there be pores in the size range of 200-600 nm.

  Most pores are preferably in the range of 25-750 nm. Naturally, the firing of diatoms does not have a significant effect on the pores present. The calcination process is carried out at a temperature in an atmosphere designed to not destroy the shell porous.

In terms of micropore surface area, it is preferred that the carbon-coated diatomaceous structure have a surface area of at least 8 m ² / g, for example at least 10 m ² / g. Larger surface areas are preferred. The potential upper limit is 30 m ² / g.

  Thus, the anode of the present invention relies on a porous silicon dioxide network coated with a carbon coating. The porous silicon dioxide network is obtained by calcination of diatoms. The coating usually consists of amorphous carbon, which is also formed during the calcination process from the carbon source present. Thus, the present invention relates, in another aspect, to an anode comprising the calcined product of diatoms in the presence of a carbon source.

  In the remainder of this application, the term "diatoms" is used to refer to calcined diatomaceous material which forms the porous silicon dioxide network used in the anode of the present invention.

<Carbon coating>
It is essential that the diatoms used in the anode structure be coated with carbon. Carbon can be obtained from carbon naturally present in diatoms or from external carbon sources, or both. The coating is preferably amorphous carbon formed from organic carbon-containing precursors such as sucrose, starch and the like. The fact that the coating is amorphous can be seen by XRD and TEM (transmission electron microscopy).

  10 to 40 wt%, preferably 15 to 35 wt%, eg 15 to 25 wt% of the diatom / coating structure, based on the combined weight of the calcined diatom and the coating, preferably 15 to 25 wt%, ideally It is preferable to make up 17 to 22% by weight. Of course, since the carbon source usually contains non-carbon elements, the amount of carbon source needed to achieve a coating level of 20% by weight is considerably more than 20% by weight of the prefired mixture May be One skilled in the art can analyze the carbon content of the carbon source to determine the amount of carbon source needed to achieve a particular final coating level.

  The carbon coating preferably forms a continuous coating on the nanoporous silicon dioxide diatom structure. The amount of carbon necessary to form a continuous coating is determined by the porosity of the diatoms.

  Naturally occurring carbon in diatoms refers to carbon sources such as other microorganisms originally present in the diatoms or in some cases on the diatoms. For example, planckton grows on diatoms and is a carbon source. Another algal source is a carbon source, ie, algae that have a carbon-based cell wall rather than a putamen. During calcination, the carbon source is converted to amorphous carbon and non-carbon atoms are liberated.

  By an external source of carbon is meant a carbonaceous material added to the diatom to form a coating on the diatom. External sources may be added in place of or in addition to the natural carbon present.

  Any carbon source can be used during the coating process. Carbon compounds are generally converted to amorphous carbon coatings during the calcination process. The carbon source is preferably a natural source such as saccharides, oligosaccharides or polysaccharides. The preferred source is a cellulosic material, such as starch.

  The carbon coating improves the performance of the anode in the battery by increasing the conductivity of the material. In addition, carbon coatings prevent particle growth during firing as well as potential particle growth and destruction of the porous network during battery cycling. The carbon coating also promotes the formation of a stable solid electrolyte interface (SEI) layer between the anode and the electrolyte. SEI is a very thin layer formed on a lithiated anode and occurs upon decomposition of the electrolyte. The SEI forms a layer that consists of organic and inorganic species and is ionically conductive but electrically insulating. This will passivate the anode, prevent further reaction with the electrolyte and allow reversible operation of the device.

  Of course, while the battery is charging and discharging, Li ions present in the electrolyte need to be inserted and extracted (extracted) to the anode. However, it is crucial that the other components of the electrolyte not co-intercalate (do not co-insert), otherwise the anode will absorb the wrong material and can not function efficiently. Surprisingly, the carbon coating on diatoms has been found to protect the anode from damage by other components in the electrolyte, as it acts to form a stable SEI layer.

  Although the thickness of the coating on calcined diatoms varies, the range of possible thicknesses is less than 10 nm. Due to the varying thickness, the use of weight percent is preferred to define the amount of coating present.

<Binder>
In order to prepare a composite suitable for the production of the anode of the present invention, it is necessary to combine the calcined diatomaceous material with its carbon coating and conductive filler such as carbon black conductor particles. The binder, when conductive, can also act as a current collector. Usually, the binder does not conduct, so a separate current collector is required. The binders used in the present invention are water soluble / water dispersible.

  The term "water soluble" means a material that is at least 1 g / L water soluble at 23 ° C. It will be appreciated by those skilled in the art that the binders of the present invention may contain minor amounts of water insoluble impurities and the like as they are derived from natural sources. Usually, these will be less than 2% by weight of any binder used.

  From another point of view the binder is an organic binder, ie the binder is derived from naturally occurring organisms such as algae, crustaceans, fungi and the like. Thus, during the preparation of the composite, the binder is supplied in aqueous form, such as a solution, dispersion, suspension, and the like. It will be appreciated that small amounts of organic solvents and water may also be present, such as less than 10% by weight, based on water. The solvent is usually polar and includes, for example, an alcohol.

  The binders used in the present invention are not PVDF based and are not based on other binders which only dissolve in NMP. It is clear that NMP is a volatile, flammable, explosive, reproduction toxic substance and it is beneficial to avoid its use. NMP is also expensive. NMP is currently used because it is the only known solvent capable of sufficiently dissolving the PVDF binder without affecting any of the other components of the electrode. The binders of the invention are introduced using water and are not non-aqueous solvents.

  Thus, the term aqueous binder means that the binder used to make the composite of the present invention is soluble / dispersible in water. It will be appreciated that the components of the composite are mixed and formed into a slurry and then cast to form a film on a support and then dried. Thus, the final composite after drying contains minimal or no water.

  The binder may be provided in an aqueous composition. The relative weight of water to binder may be in the range of 2: 1 to 100: 1, such as in the range of 10: 1 to 75: 1. The amount of water can vary over a wide range depending on the nature of the slurry required. For example, if it is desired to increase the viscosity of the slurry, less solvent is added to the binder and vice versa. Of course, this ratio depends on the solubility / dispersability of the binder.

  The binders used in the present invention are preferably based on saccharides such as oligosaccharides or polysaccharides such as cellulosics. Thus, the binder may be water soluble chitin, alginate, water soluble chitosan and the like. Preferred binders are alginates or water-soluble chitosan, in particular alginates, such as sodium alginate. It will be appreciated that aqueous compositions of these types of substances may swell.

The alginate binder may be alginate, alginic acid, or a salt of alginic acid. Furthermore, the salt of alginic acid may be a salt of Na, Li, K, Ca, NH ₄ , Mg or Al of alginic acid. The molecular weight of the alginate containing composition may be from about 10,000 to about 600,000.

  The use of alginate is particularly preferred as alginate is obtained from algae. Furthermore, the carbon coatings on diatoms are often derived from sugar structures such as starch. This means that the two components are inherently compatible. In order for the anode to function, it is important that the binder wet the surface of the carbon coated diatom and that wetting be maximized by the intrinsic compatibility of the binder material with the carbon coated diatom.

  The aqueous binder of the present invention is non-toxic, environmentally friendly and exhibits excellent properties, for example, in terms of cycle characteristics, stability, life and rate performance. This binder can also provide an ideal support to prevent SEI degradation.

  In a further embodiment, it is possible to add carbon nanotubes (CNTs), other forms of conductive carbon additives or other carbon-free conductive additives to the binder in order to maximize the performance of the binder. is there. These may comprise up to 30% by weight of the binder, for example 5 to 20% by weight (calculated on dry weight).

  Particularly preferred is the combination of alginate and CNT, which combination forms a new material for the binder in LIB. This forms a further aspect of the invention. Thus, the present invention provides, in another aspect, a binder suitable for use in LIBs comprising alginate and carbon nanotubes.

<Conductive filler>

The anode of the present invention is also generally particulate and also contains a conductive filler such as carbon black. Although any conductive filler may be used, the filler is usually carbonaceous such as carbon nanotubes or preferably carbon black. Carbon black is a conductor and is used to enhance the conductivity of the composite electrode by providing a conductive path between the silica shell and between the silica shell and the current collector.

  The carbon black of the present invention, as measured using transmission electron microscopy as described in ASTM D 3849-95a, Dispersion Procedure D, may have an arithmetic average primary particle size of at least 20 nanometers and up to 70 nanometers.

More preferably, the carbon blacks of the present invention, when measured according to ^{ASTM D 2414-06a, 80~300cm 3 / 100g} , preferably has a 180cm ^3/100 g of less than DBP (dibutyl phthalate) Number of absorption.

  Non-limiting examples of the carbon black of the present invention are, for example, carbon black grades indicated by "ASTM Nxxx" code, such as Ensaco black, supplied by Timcal, Timcal (Timcal). And the like) acetylene black, furnace black and ketjen black. Preferred carbon blacks are, for example, Ensaco black, furnace carbon black supplied by the company Timcal.

The surface area of the carbon black (CB) can be up to 80 m ² / g, as measured according to ASTM D 4820-99 (BET, N ₂ adsorption). The ideal surface area is 30-80 g / m ² .

  Mixtures of calcined carbon-coated diatoms, fillers such as binders and carbon black particles are referred to herein as composites. This composite is used to form an anode, as described further below.

  The amount of filler, such as carbon black, present in the composite of the present invention is preferably 15 to 50% by weight, such as 25 to 45% by weight, in particular 30 to 40% by weight (dry weight).

  The amount of binder present in the composite (the term binder refers to the weight of the total dried binder, eg including all CNTs when present) is 2.5 to 30% by weight, eg 5. Such as 0-25% by weight, in particular 7.5-20% by weight.

  The amount of diatoms present in the composite (the carbon coating forms part of the diatoms) is 30 to 80% by weight, such as 35 to 70% by weight, in particular 40 to 60% by weight (dry weight).

  The term "dry weight" being used means that the percentage is based on the absence of moisture. Ignore the water present.

<Method>
In order to manufacture the anode of the present invention, a diatom source is required. Diatoms can be collected from any source, such as the ocean. It is preferred to clean or wash the diatoms prior to use to produce the composite of the present invention. The collected diatoms can be dried, clarified and dried again or simply cleaned and dried according to the following procedure. Both processes are effective. However, as discussed below, the purification process is very important to performance. If the diatoms are not properly cleaned prior to firing, this leads to a reduction in overall capacity.

  Thus, in one embodiment, the diatom source is first dried. Any drying conditions can be used, including, for example, reduced pressure, elevated temperature, and the like. Ideally, the diatoms are brought to a temperature above the boiling point of water to remove the water. However, the temperature is not high enough to destroy diatoms or high enough to destroy naturally occurring carbon in diatoms. A range of 110 to 200 ° C., such as 115 to 150 ° C., is suitable.

  After drying or harvesting, it is preferred that the diatoms be well washed, depending on the process used. Surprisingly, it has been found that the salt present in the collected diatom as it is present in seawater prevents the anode from functioning ideally. It is an important step as well cleaning removes these salts. Thus, the washing step removes metal salts such as sodium chloride or KCl, and any process that does not damage diatoms while removing these salts is suitable.

  For example, it may be washed in dilute acid. It is also possible to wash in water, for example repeatedly in water.

  In a preferred embodiment, the amount of water present is: diatoms: water (wt%) 1:50 to 1: 150, such as 1:70 to 1: 100. After initial mixing, it is preferred to raise the temperature of the water to a temperature slightly below the boiling point of water, eg up to 90 ° C. The temperature can be raised above the boiling point of water but below 150 ° C., for example to 115-140 ° C. under reflux. The material can be refluxed / held just prior to boiling for a period of time, for example 2 hours. The temperature of the material is then reduced to a temperature below the boiling point, for example 50-90 ° C. This material can be held at this temperature for an extended period of time, for example 4 hours.

  The material may then be sieved and washed once more in water and then again in running water. The entire procedure may then be repeated.

  After completion of the washing procedure, the resulting diatom product is dried. The purified and dried diatoms have a much reduced content of metal salts such as KCl and NaCl. Before purification, the metal salt content of diatoms may exceed 1.5% by weight, but the metal salt content of the purified material is preferably less than 1% by weight, ideally less than 0.5% by weight , In particular less than 0.1% by weight. The most abundant salts present are KCl and NaCl. More specifically, the total content of KCl and NaCl salt of the purified material is preferably less than 1% by weight, ideally less than 0.5% by weight, in particular less than 0.1% by weight. Ideally, diatoms do not contain metal salts, in particular KCl and NaCl. "Not included" means that the peaks of these substances are not seen in XRD.

  If the carbon naturally contained in diatoms is not removed at this stage, for example, if no external carbon source is added, it is preferable to bake the dried and washed diatom material. The calcination is preferably performed at a temperature such as 400-800 ° C., such as 500-750 ° C., ideally 550-700 ° C., such as about 600 ° C. or 650 ° C. The baking treatment is performed in an inert atmosphere, that is, an atmosphere containing no oxygen in order to prevent an oxidation reaction. Any inert medium such as nitrogen or a noble gas such as Ar may be used.

  If the firing temperature is too high, it may be possible to destroy the crustal nanostructure. If the firing temperature is too low, the carbon source will not form a coating on the diatoms.

  At this stage, it is also possible to mix diatoms with an external carbon source and carry out a baking treatment. The following describes the addition of the carbon source, and the same principle applies to the addition of the carbon source at this stage.

  After firing, the resulting material is a porous silicon dioxide structure, eg, a nanoporous silicon dioxide structure coated with carbon, ideally amorphous carbon.

  In another embodiment, any naturally occurring carbon can be removed from diatoms. This makes it possible to control the carbon content in the final material fairly well, since only the carbon present is added. After purification of the diatoms, it is preferable to bake the diatoms in the presence of oxygen rather than firing. The temperature of this baking step is preferably the same as the baking temperature, for example 400-800 ° C., such as 500-700 ° C. as an example, ideally about 600 ° C. In the presence of oxygen, any carbon source present, such as plankton, is destroyed leaving only the upper layer structure of porous silicon dioxide diatoms.

  At this point, an external carbon source can be added. Although any carbon source may be used, ideally the carbon source is a saccharide, an oligosaccharide or a polysaccharide. Thus, the carbon source may contain only C, H and O atoms. For example, cellulosic materials such as starch based products are suitable. The carbon source can be added alone or in a solvent such as water. Thus, an aqueous carbon source can be added to the baked diatoms to provide a carbon source, which will be the desired carbon coating on the diatoms during the subsequent firing step. Thus, after the carbon source is supplied, the diatom / carbon source is calcined as described above.

  The firing step produces a porous carbon coated diatomaceous structure. As a further option, it is possible to use both an external source of carbon and carbon originally present in diatoms. Thus, a carbon source may be added after the purification step and before calcination.

  It should be noted that the thickness of the carbon coating on diatoms can be adjusted by varying the amount of carbon source present. The greater the amount of external carbon source, the thicker the coating. Preferably, the carbon content is 10 to 35% by weight, preferably 15 to 25% by weight and ideally 17 to 22% by weight carbon of the diatom / coating structure, based on the total weight of diatoms and carbon source .

  FIG. 1 contains two non-limiting process diagrams illustrating the process of the present invention suitable for the production of carbon coated diatoms of the present invention.

Thus, in another aspect, the present invention
(I) procuring a diatom source, for example, from the ocean, and optionally drying the diatom;
(Ii) washing the diatoms and optionally drying the washed diatoms;
And (iii) calcining the product of step (ii) under an inert atmosphere at a temperature of 400-800 ° C. to provide a method of making a porous carbon-coated silicon dioxide network.

  Optionally, an external carbon source can be added after step (ii).

The invention, in another aspect, comprises
(I) procuring a diatom source, for example, from the ocean, and optionally drying the diatom;
(Ii) purifying the diatoms and optionally drying the purified diatoms;
(Iii) Baking the diatom of step (ii) at a temperature of 400 to 800 ° C. in an atmosphere containing oxygen,
(Iv) adding a carbon source to the baked diatoms of step (iii);
(V) calcining the product of step (iv) at a temperature of 400-800 ° C. under an inert atmosphere, to provide a method of making a porous carbon-coated silicon dioxide network.

Once the carbon coated diatoms are prepared, they can be mixed with a filler such as carbon black and a binder to form a composite. The components are ideally mixed in water to form a slurry. The slurry can be tape cast onto a support, such as a current collector (eg, Cu). The thickness of the film to be formed can be adjusted and adapted to the needs of the cell in question, but its thickness is usually 10 to 100 microns, such as 50 to 60 μm. From another viewpoint, the supported amount on the support, 1-15 mg / cm ^2, preferably may be 1-10 mg / cm ^2. After drying, this material is suitable for use as the anode of the cell of the invention.

  Accordingly, the present invention relates to the use of the composite of the present invention as an anode in a Li-ion battery and to a Li-ion battery comprising the above-described composite.

Lithium ion batteries using the composite of the invention as the anode are otherwise conventional. They include, as is known, a cathode and an electrolyte, a separator and a current collector. Electrolyte used comprises LiPF _6. The cathode is preferably a Li-containing transition metal oxide. Li-based compounds such as LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (NMC-111), LiCoO ₂ (LCO), LiNi _0.8 Co _0.15 Al _0.05 O ₂ (NCA) and LiFePO ₄ (LFP), It can be used as a positive electrode (cathode). It is preferable to use a polymer membrane, such as a polypropylene membrane, as a separator or the like. Further discussion of other conventional features of these batteries is not necessary.

  A further advantage of the present invention is that over time, part of the porous silicon dioxide network obtained by calcination of diatoms is reduced to Si. As mentioned above, Si is a very interesting anode material for Li-ion batteries because of its very good theoretical performance. However, absorption of ions leads to very large expansion problems. In the present invention, the silica is slowly reduced to Si as the battery is used and charge and discharge cycles are repeated. Due to the relatively low amount of Si formed over time (possibly less than 12% by weight of the total silica content) and the porous structure of the silicon dioxide network, the anode of the invention results in a small amount Can cope with the expansion of Thus, the inclusion of Si provides an anode that has excellent performance while avoiding expansion problems that make current Si anode solutions difficult.

  The battery of the invention exhibits valuable properties in terms of capacity. The capacity is the amount of charge that the battery can supply, and is expressed as mAh / g. The battery of the present invention may have a capacity of at least 700 mAh / g, preferably at least 800 mAh / g, after 50 cycles of charge and discharge. These values are measured with an applied voltage window of 0-2.5V.

  The absence of a loss of capacity over time (e.g., up to at least 100 cycles) is an important characteristic of the batteries of the present invention. After repeating the cycle, a reduction of less than 2% is observed. Preferably, an improvement in performance is observed by forming a limited amount of Si.

  The invention will now be described with reference to the following non-limiting examples and figures.

FIG. 1 shows a flow chart of the preparation of the composition of the invention.
FIG. 2 shows the X-ray diffraction pattern of diatoms. (A) Baking at 600 ° C. in argon atmosphere without purification step, (b) Baking at 1000 ° C. in argon atmosphere without purification step, (c) Baking at 650 ° C. in air after washing step, and (d) After the washing step, bake at 600 ° C. in argon.
FIG. 3 shows a TG curve of carbon-coated diatom (SiO ₂ -C).
FIG. 4 shows the relationship between the residual carbon content of diatoms after calcination and the added starch content.
FIG. 5 shows FESEM images of carbon coated diatoms (SiO ₂ -C).
FIG. 6 shows the pore size distribution and pore surface area of carbon-coated diatom complex (SiO ₂ -C) measured by mercury intrusion porosimetry (MIP). FIG. 7 shows the fines of washed diatoms after firing in air at 600 ° C. for 2 hours and washing, and after firing carbon-coated diatoms (SiO ₂ -C) calcined in argon for 2 hours at 600 ° C. 7 shows BET surface area including pores / outer surface.
FIG. 8 shows constant current charge / discharge profiles of carbon-coated diatom-based anodes (SiO ₂ -C) in the first, second, twenty-fifth and fifty-fifth cycles.
FIG. 9 shows the percent irreversible capacity and specific capacity at current densities of 50 mA / g and 100 mA / g as a function of cycle number for carbon-coated diatom-based anodes (SiO ₂ -C).
FIG. 10 shows the differential capacity plots of the carbon-coated diatom-based anode (SiO ₂ -C) at cycles 1, 25 and 50.
FIG. 11 shows a comparison of the specific capacities of a commercial graphite anode and a carbon-coated diatom-based anode (SiO ₂ -C) at moderately high current densities (100 mA / g) using various potential windows.
FIG. 12 shows the constant current charge / discharge specific capacity of carbon coated diatom based anodes (SiO ₂ -C) at different current densities (100, 200, 500, 1000 mA / g).
FIG. 13 shows a carbon-coated diatom base based on carbon coatings obtained from different amounts of starch sources (0, 35 and 80 wt%) at low current densities (20 mA / g) using voltage windows (0 to 2.5 V) A comparison of the specific capacity of the anode of
FIG. 14 shows the total specific capacity of Li-ion batteries as a function of anode specific capacity (C _A ). Consider cathodes with specific capacities of 100, 150 and 200 mAh / g.
FIG. 15 shows a comparison of the total specific capacity of Li-ion batteries based on Equation 1 with diatom-based anode (SiO ₂ -C) and graphite anode, (a) theoretical specific capacity of graphite (372 mAh / g And (b) using the experimentally measured specific capacity of graphite (80 mAh / g) at a current density of 100 mA / g. Cases (FIGS. 15a and FIG. 15b) is also the specific capacity of the current density experimentally measured diatom based anode at _{100mA / g (SiO 2 -C)} (850mAh / g) is used.
FIG. 16 shows (a) constant current charge / discharge specific capacity of a graphene-based anode at different current densities (100, 200, 500, 1000 mA / g) and (b) graphene (at 100 mA / g current density) 225 mAh / g) and a comparison of total specific capacity of Li-ion batteries based on equation 1 using the experimentally measured ratio volume of diatom based anode _{(SiO 2 -C) (850mAh /} g).

[Example]
Example 1
Preparation of SiO ₂ -C Composite (Porous Silicon Dioxide Network with Carbon Coating)
Diatom-based anodes were prepared using two different routes (see FIG. 1). In Route 1, the organic matter present in the diatoms was used to coat the diatoms to prepare a carbon-coated SiO ₂ composite. In Route 2, starch was used as a source of carbon to coat diatoms during the process. The preparation procedure for both routes is described in detail below.

Route 1: Diatoms are a major group of algae that grow the cell wall of silica (SiO ₂ ), called the putamen, and were collected from the northern Norwegian Sea by Planktonik As. These putamen (mainly SiO ₂ ) provide natural nanostructured porous materials. The as-received diatoms were dried at 120 ° C. for 36 hours. The dried diatoms were then clarified (procedure described below) to remove various types of salts present. The diatoms were then calcined at 600 ° C. for 2 hours in an argon atmosphere. The organics present in the diatoms served as a carbon source to provide a coating of carbon on the diatoms. The treated carbon-coated SiO ₂ composite is denoted as SiO ₂ -C.

Example 2
Route 2: A clean diatom sample was baked at high temperature (600 ° C.) for 2 hours in synthetic air using Route 2 to prepare another batch of composite. In this step, all the organic substances present in the diatoms were decomposed, and the SiO ₂ -based porous structure remained. This nanostructured porous diatom (mainly SiO ₂ ) was then mixed with 35-80 wt% corn starch as a carbon source and heat treated at 650 ° C. for 2 hours in an argon-filled inert atmosphere. The sample treated according to this route is carbon-coated nanostructured porous SiO ₂ , and the thickness and amount of the coating depend on the amount of corn starch mixed as a carbon source. Both baking and baking were performed using a horizontal tube furnace (Carbolite Ltd., Sheffield, UK). The treated carbon-coated SiO ₂ composite _shows the SiO 2 -C (Stxx). The percentage of starch mixed with the SiO ₂ -C complex is described in the location of xx in Stxx (for example, a composite to which 50% by weight of starch has been added is indicated as SiO ₂ -C (St 50) ).

[Example 3]
<Diatom purification procedure>
The dried diatoms were rinsed and placed in a large amount of deionized water at room temperature to maintain a weight ratio of diatoms to water of 1:70 to 1: 100. The temperature of the water was raised to 90 ° C. while stirring at 400-500 rpm for 2 hours. The temperature of the water was lowered to 80 ° C. in 4 hours while stirring at the same stirring speed. The hot water was drained using a sieve with a mesh size of 32-63 μm. Fresh deionized water was added to the sample, keeping the weight ratio 1:50, and sonicating for 0.5 hours. The diatoms were then washed under running water for 5-10 minutes. Then this process was repeated. After washing, the sample was dried at 90 ° C. for 24 hours in a drying oven. After removing the water, the sample was vacuum dried at 150 ° C. for 18 hours. These purified diatoms are used for further processing to prepare nanostructured SiO ₂ -based anodes.

Example 4
<Preparation of sodium alginate-based aqueous binder>
Alginic acid binder is made from sodium salt of alginic acid (Na-Alg) extracted from cell walls of brown algae. The ratio of Na-Alg (weight, g), deionized water (volume, ml) and ethanol (volume, ml) was maintained at 1: 60: 6. In the case of a carbon nanotube (CNT) -added binder, the ratio of Na-Alg (weight is g), deionized water (volume, ml), ethanol (volume, ml) and CNT (weight, g) is 1:60 6: 6: 0.2. First, sodium alginate was added to ethanol with stirring. The temperature of the mixture was then slowly raised to 80.degree. Deionized water was added to the mixture when the temperature reached 80.degree. After adding deionized water, the temperature was maintained at 80 ° C. for 10 minutes. The vessel is then covered and stirred at 500 rpm for 18 hours at 50 ° C.

  In the case of a carbon nanotube (CNT) added binder, CNT is added after this step. The temperature is reduced to 30 ° C. while maintaining a stirring speed of 500 rpm. Stirring was stopped when the CNTs were thoroughly mixed with the alginate binder solution for about 6-8 hours.

  The ideal mixing method is to sonicate the CNTs with deionized water for 20-30 minutes for proper dispersion and then add to the solution when the temperature of sodium alginate in ethanol reaches 80 ° C. is there. After addition of the dispersed CNTs in deionized water, the temperature was maintained at 80 ° C. for 10 minutes. The vessel is then covered and stirred at 500 rpm for 18 hours at 50 ° C.

<Characteristics evaluation>
The SiO ₂ -C composite obtained by processing the diatoms were analyzed by X-ray diffraction (XRD). A Bruker AXS D8 FOCUS diffractometer was used, equipped with a one-dimensional LynxEye PSD detector and a 40 kV, 40 mA Ni filter CuKα line at 1.5 mA and wavelength 40 mA. Scans were recorded at a step rate of 0.1 ° with a 2θ value of 10-80 ° at a scan rate of 2 seconds / step. The specific surface area of the composite was measured by nitrogen adsorption measurement (Tristar 3000 Micrometrics). Samples were degassed at 150 ° C. for 24 hours prior to analysis. 54 and 40 points were measured for the adsorption and desorption isotherms, respectively. The T-plot theory based on BET theory was applied to the distinction between the microporous area and the outer surface area. Bulk density, porosity and pore distribution data were obtained by using mercury intrusion porosimetry (Micromeritics, Auto pore IV 9500) over a pressure range of 0.10 to 60000 psia. The contact angle was assumed to be 130 ° in the calculation of the pore size. Thermogravimetric analysis (TGA) of the as-prepared SiO ₂ -C complex is carried out using a Netzsch STA 449 C Jupiter (Selb, Germany) in synthetic air at a heating rate of 10 ° C. min ^-1 up to 850 ° C. did. The TG curve was recorded under a synthetic air flow of 30 mL min ^-1 . In all cases, the reference value was subtracted. The morphology of the product was observed using a field emission scanning electron microscope (FESEM, Zeiss Supra-55 VP).

[Example 5]
<Anode>
Electrochemical characteristics of SiO ₂ and SiO ₂ -C composites using CR2016 coin cell assembled in argon filled glove box (Labmaster SP, M. Braun GmbH, Germany) with water and oxygen concentration of 0.1 ppm Was evaluated. The coin cell is composed of lithium as a counter electrode, the working electrode is 35% by weight of Super P carbon black and 15% by weight of aqueous binder (alginate Alg: water = 1: 60 by weight) and 50% by weight of SiO ₂ Prepared by mixing -C complex. Therefore, assume that you target the following composites:
SiO2-C composite material 50 wt% = 0.30 g
Carbon black = 35% by weight 0.21 g
Alginate = 15% by weight = 0.09g

The binder solution consists of binder plus solvent, and there is 60 times the water content of alginate, ie, = 0.09 x 60 = 5.4 g of binder solution. A high binder content was recommended in silicon based electrodes for excellent cycleability. The electrodes were formed by tape casting the slurry onto battery grade Cu foil (18 μm, Circuit Foil Luxembourg) followed by drying overnight at 90 ° C. in a vacuum oven. A 25 μm microporous single-layer polypropylene-based membrane (Celgard 2500, USA) was used as a separator. The electrolyte used was 1 M LiPF 6 (Sigma-Aldrich, 99.99%) dissolved in ethylene carbonate / diethyl carbonate (1: 1 volume ratio). Charge / discharge analysis was carried out at a constant current of 0 to 2.5 V at room temperature. All volumes reported are described in terms of the mass of the SiO ₂ -C complex.

<Results and Discussion>
<Characterization and process development of the SiO ₂ -C composite>
X-ray diffraction of diatoms (SiO ₂ ) was performed at different stages of the process to identify the phases of silica and other minerals present. The XRD pattern of the diatoms calcined at 600 ° C. for 2 hours without washing shows the peaks identified as NaCl and KCl (see FIG. 2a). A sharp peak was seen when the temperature rose to 1000 ° C. in 2 hours (see FIG. 2 b). Diffraction lines at 2θ = 21.8 °, 28.5 °, 36.2 ° [JCPDS 01-0438, 03-0267] are attributable to cristobalite. However, cristobalite diffraction lines are not present for diatom samples calcined at 600 ° C. (see FIG. 2 a). FIG. 2 c shows the XRD pattern of washed diatoms calcined at 650 ° C. in air. There are no diffraction lines due to crystalline salts (NaCl and KCl). Thus, the purification procedure completely removes the salt present prior to purification.

  In FIG. 2c, there is no clear diffraction line at all because the diatoms are amorphous. The strong signal from the crystalline salt seen in FIG. 2a conceals that from amorphous silica, which can be seen in FIG. 2c.

  All organics are burned out at this high calcination temperature, leaving only the nanostructured silica. The center of the spectrum is at about 2θ = 22 °, which is characteristic of amorphous silica. The smoothness of the pattern also indicates that the purification process with multiple washes effectively removes the NaCl and KCl salts trapped in the diatom porous structure.

FIG. 2 d shows the XRD pattern of carbon coated diatoms (SiO ₂ -C). Only the salt is removed in the purification step, and the organic matter present in the diatoms is used to coat the diatoms. The broad features seen in FIG. 2 d indicate that both the carbon layer and the diatom (SiO ₂ ) are amorphous, and no other crystalline impurities are present in the carbon coated diatom (SiO ₂ -C). It also supports that.

<Thermogravimetric analysis (TGA)>
Using thermogravimetric analysis to determine the content of carbon coated on diatoms (SiO ₂ -C and SiO ₂ -C (Stxx) composites). Figure 3 shows a thermogravimetric curve obtained for SiO ₂ -C composite residual organic substances in the diatoms to provide a coating used as a carbon source. One-step characteristics can be seen from the TGA curve. The initial slight weight loss below 200 ° C. is due to the elimination of residual water vapor and the evaporation of gaseous components.

The carbon content of SiO ₂ -C is obtained, assuming that a weight loss between 375 and 600 ° C. results from the oxidation of carbon to CO and CO ₂ in constant flow of oxygen. For SiO ₂ -C composite, weight loss of about 17 wt% carbon is measured.

In the case of the composite SiO ₂ -C (Stxx), corn starch is used as a carbon source to coat organic-free diatoms. The starch content used for the preparation of the samples varied between 35 and 80% by weight. A linear relationship is observed between the measured carbon content and the amount of corn starch used (see FIG. 4). Thus, it is possible to estimate the amount of corn starch needed to provide a constant amount of carbon coating on diatoms.

<Form>
In field emission scanning electron microscopy (FESEM), standard micro- and nano-scale microstructures of carbon-coated diatoms (SiO ₂ -C) with repeated changes in pore size distribution were observed. The observed porous morphology has a very narrow pore size distribution and long range order (see FIG. 5). Diatoms are divided into more than 200 families. From FIG. 5 (a, b and c), the diatoms transported from the northern part of the Norwegian Sea are disk-like and identified as coschinodiscus coskininodiscus. This type of diatom is usually considered to be one of the largest marine diatoms. It was observed that valves of various shapes and sizes were repeated (Fig. 5 (d-o)). The pore size and pore distribution of carbon-coated diatoms (SiO ₂ -C) were observed in detail by mercury intrusion porosimetry.

The porosity, pore size distribution and pore surface area of carbon-coated diatom complex (SiO ₂ -C) were measured using mercury intrusion porosimetry (MIP). In FIG. 6, the x-axis represents the pore size on a logarithmic scale. The y-axis (dV / dlogP) represents the change in volume of mercury intruded against the logarithm of the pressure required to drive mercury into the pore. The red line indicates the cumulative pore area, and since mercury was gradually taken in, it was rapidly taken in because a large number of pores were packed at one time. As can be seen from FIG. 6, there are a plurality of peaks in the range of 50 to 600 nm, and the pore size distribution is very narrow at each peak. These peaks corresponded to mesopores and macropores and contributed by the surface area. As the pore size increases, the surface area decreases. Small pores contribute more to surface area than large pores. It is noteworthy that the sharp big peak of about 50 nm corresponds to the incorporation of higher amounts of mercury and contributes significantly to the surface area. Therefore, it can be inferred based on the peak shape that the diameter of many pores of diatoms is in the range of 50 to 80 nm, while the diameter of other pores is in the range of 200 to 600 nm. The results obtained from mercury intrusion porosimetry were also corroborated by morphological observations made with a field emission scanning electron microscope (see FIG. 5).

The surface area and porosity of the carbon coated (SiO ₂ -C) and uncoated diatoms (SiO ₂ ) were determined by BET analysis based on nitrogen adsorption. BET is most suitable for the study of micro (<2 nm) and meso (2-50 nm) pores. Depending on the material, it may be difficult to generate comprehensive data on the distribution of pores above 100 nm. On the other hand, nitrogen adsorption can provide useful information on the micropores arising from amorphous carbon coatings. As expected, no pore area was detected for the samples without carbon coating. However, the external surface area was about 12 m ² / g. For the carbon coated diatom (SiO ₂ -C) sample, the detected micropores and external surface area were 19 m ² / g and 12 m ² / g, respectively. On the other hand, the pore surface area measured by mercury porosimetry for the same sample was about 10 m ² / g. The pore surface area values generated by the indentation method are generally found to be consistent with gas adsorption if the pores are very small, i.e. <10 nm. These pores, if present, are outside the range of measurement by the indentation method, so their surface area values are unnaturally low. The external surface area of the coated diatoms and the uncoated diatoms was found to be equal. This confirms the assumption that the carbon coating produces micropores in the diatom-based sample.

<Constant current cycle>
In a two-electrode coin cell (half cell) configuration using a diatom-based composite as the electrode material and lithium foil as the counter electrode, the electrochemical behavior of all the anodes in this study during charging and discharging was investigated. Thus, for diatom-based electrodes in use, the term charging means dealloying and the term discharging means alloying. FIG. 8 shows the cycle potential window (0-2.5 V) versus the charge / discharge specific capacity of a 50 wt% carbon-coated diatom-based composite (SiO ₂ -C) electrode at a current density of 50 mA / g . The first discharge cycle (alloying) gives a specific capacity of 1050 mAh / g. In the second cycle, the capacity dropped to 800 mAh / g and began to increase in successive cycles. It reached 875 mAh / g in the 50th cycle. However, the specific capacity during charging also increases from the second cycle (800 mAh / g) to the 50th cycle (865 mAh / g). The irreversible capacity loss in the first cycle is due to more Li ⁺ ions trapped inside the diatom pore or consumed at the solid electrolyte interface (SEI) formation. It is also apparent from the charge curve pattern (see FIG. 8) that de-alloying mostly occurs at a potential of <0.5 V and alloying occurs at a potential of <0.25 V (versus Li / Li ⁺ ).

Constant current charging / discharging cycles of carbon-coated diatom-based composite (SiO ₂ -C) electrodes at current densities of 50 mA / g (1 to 30 cycles) and 100 mA / g (31 to 96 cycles) . The capacity is gradually increased until it stabilizes in about 50 cycles (see FIG. 9). An increase in capacity is observed at any current ratio and is more pronounced during the first 20 cycles. This capacity increase is due to the increase in the amount of silicon (Si) phase as the silica (SiO ₂ ) is partially reduced by Li, and the capacity increases with time as more silicon is converted from the silica It is believed that. The differential capacitance versus voltage curve is shown in FIG. 10 for the different cycles measured at a current density of 100 mA / g. The phenomenon that SiO ₂ is reduced to Si or crystalline Li ₄ SiO ₄ during an initial discharge (alloying) of less than 0.24 V is widely known. The reduced Si further reacts with lithium ions as the potential approaches 0.0 V to form a Li _x Si alloy. The formation of the Li ₄ SiO ₄ phase during the cycle is irreversible. A surge of capacity is observed due to the capacity obtained by including in the structure a new Si phase in excess of the irreversible formation of the Li ₄ SiO ₄ phase during the initial few cycles. A noticeable peak is observed during the charge cycle at about 0.3 V, which can be attributed to the dealloying of the Li _x Si phase. Sharpening of the dealloying peak English and sharpening indicate that the kinetics of delithiation is promoted as the number of cycles increases.

  The irreversible capacity loss shown in FIG. 9 decreases with further cycling and remains stable at less than 1%. This irreversible capacity is inversely proportional to the coulombic efficiency. Thus, although the coulombic efficiency increases with the number of cycles, it is in good agreement with the differential capacity curve of FIG.

A comparison of the constant current charge / discharge cycles of carbon-coated diatom-based composite (SiO ₂ -C) and graphite for 50 cycles at a current density of 100 mA / g at two different potential windows is shown in FIG. When the voltage window is maintained between 0 and 2.5 V, stable specific capacities of over 850 mAh / g are obtained with carbon-coated diatom-based composites (SiO ₂ -C). As conventional graphite electrodes operate at a potential only 150 mV higher than lithium metal, 0-2.5 V is too high as an active potential window for graphite. Therefore, lower potential windows (0-1 V) are considered more realistic to compare their capacitance values. In the carbon-coated diatom-based composite (SiO ₂ -C), a stable specific capacity of about 550 mAh / g was clearly obtained, while under the same cycling conditions, the stable specific capacity of the graphite-based electrode was It was found to be about 70 mAh / g. Direct quantitative comparison can not be deduced from the results obtained, as the active material masses at the two electrodes are different. However, the specific capacity of the composite (SiO ₂ -C) based on carbon-coated diatoms is four to five times the specific capacity of graphite under the described conditions, even when considering the equivalent active mass in the calculations It is clear that there is one.

The velocity performance of the carbon-coated diatom-based anode (SiO ₂ -C) is shown in FIG. 12 at various current densities (100, 200, 500, 1000 mA / g). As the current density increases, the specific capacity decreases. At high current densities (1000 mA / g), unlike the others, the charge (de-alloying) specific capacity was clearly greater than the discharge (alloying) specific capacity (see FIG. 12). This particular behavior at high current rates can be explained by the resistance of the solid electrolyte interface (SEI). The SEI formed during the first cycle has a constant resistance to pass lithium ions. When the discharge (alloying) current exceeds a certain limit, a voltage drop occurs across the SEI, and lithium ions are deposited on the SEI instead of letting lithium ions pass (McDowall 2008). This lithium plating damages the cell and reduces its cycle life. Therefore, it is necessary to clarify the maximum discharge (alloying) current and to prevent damage and capacity reduction.

From the results obtained from the carbon-coated diatom-based anode, it is clear that when the carbon coating consists of different amounts of starch (0, 35 and 80% by weight) the cycle performance improves with increasing carbon content (See Figure 13). In the case of the composite SiO ₂ -C (St 80), a thick carbon (about 30% by weight) layer also provides a stable structure with adequate interconnected conductivity, buffering large volume changes during cycling. While thermogravimetric analysis, shown in FIG. 4, shows that only about 6 wt.% Carbon can be obtained from 35 wt.% Starch, this is insufficient to serve the purpose of demonstrating a complete carbon coating. Therefore, SiO ₂ —C (St 35) and SiO ₂ (St 00) have almost the same charge specific capacity and discharge specific capacity, and follow the same tendency. The slightly higher capacity of SiO ₂ (St 00) compared to the SiO ₂ -C (St 35) composite is due to the presence of more active material. The obtained results also indicate that the interfacial resistance of the electrode particles significantly contributes to the electrode resistance. Therefore, it is important to have a carbon coating of a certain thickness in order to take advantage of the carbon coating and to improve the electrode conductivity.

<The analogy in all cells>
Despite worldwide research efforts, the specific capacity of commercially used cathodes can not exceed the value of 200 mAh / g, mostly less than 150 mAh / g. On the other hand, practical value of the carbonaceous anode have recently reached the theoretical value of the composition LiC _6. However, these carbonaceous anodes were reduced in value because lithium ion diffusion rates could not be increased when high current densities were used. Thus, higher capacity anode materials are currently the only solution to increase the total cell capacity of the electrode material (cathode + anode). A typical relationship between the total specific capacity of the cell as a function of anode specific capacity is given by Yoshio et al. [Yoshio, Tsumura et al. 2005)], where the specific capacity (C _C ) of the cathode is constant: The total specific capacity of the cell varies with the specific capacity of the anode (C _A ).

Using the above equation (Equation 1), three different capacity (100, 150 and 200 mAh / g) cathodes were examined to create FIG. Two areas can be clearly identified. That is, the total capacity increased rapidly as the anode capacity increased initially, and then it was flattened when the capacity of the anode exceeded a certain value. Thus, the optimized capacity of the anode is specified within the range of 1000-1250 mAh / g which gives the best value for all cells. It has been found earlier in FIG. 8 that when cycled by constant current at a current density of 100 mA / g, nanostructured carbon coated diatoms (SiO ₂ -C) provide a capacity of 850 mAh / g. However, when cycling at lower current densities (eg, 50 mAh / g), the capacity exceeds 900 mAh / g.

  A comparison of the increase in total cell capacity, with the cathode capacity fixed at 150 mAh / g and the anode capacity varied with the graphite based anode and the diatom based anode, is shown in FIG. FIG. 15a shows a comparison of the 20% increased total cell capacity with respect to the experimental capacity (850 mAh / g) of the diatom-based anode measured at 100 mA / g current density and the theoretical capacity of the graphite anode (372 mAh / g). Due to the slow diffusion rate of lithium ions, it has been found that the graphite based anode can not maintain its capacity when high current density is used. This behavior of the graphite anode is in good agreement with the findings of Zaghib et al. [(Zaghib, Brochu et al., 2001)] who found that solid diffusion of lithium ions was slow as the current rate increased. This slow diffusion of lithium ions ultimately limits the insertion capacity of the graphite. Therefore, it is more practical to compare the values obtained by experimentally measuring both capacity values. Therefore, the total cell capacity is calculated based on the capacity of diatom-based anode (850 mAh / g) measured at half cell shape current density 100 mA / g and the capacity of commercially procured graphite anode (80 mAh / g) And is shown in FIG. Comparing the specific capacities of graphite and diatom-based anodes at higher current rates, the total cell volume can be increased by 20-140% when diatom-based anodes are used.

Graphene, a high capacity carbonaceous anode, was also used to compare total cell capacity with diatom-based anodes (SiO ₂ -C). Graphene was prepared in the laboratory from natural graphite using the Hummers method [Hummers and Offeman 1958]. After the formation of graphite oxide, heat treatment was performed at low temperature (at 170 ° C. for 0.3 hours) and at high temperature (at 800 ° C. for 2 hours) to obtain graphene in the final form. The theoretical capacity of graphene varied between 780 and 1116 mAh / g by Li ion absorption of Li ₂ C ₆ or Li ₃ C ₆ respectively [Hou, Shao et al. 2011], (Hwang, Koo et al. 2013)! At very low current densities, in this case 20 mA / g, the graphene cell reached a specific capacity of about 824 mAh / g. However, as the current density increased, graphene also behaved like graphite and its capacity dropped significantly. The constant current charge / discharge specific capacities of graphene based anodes at different current densities (100, 200, 500, 1000 mA / g) are shown in FIG. 16a. At 100 mA / g current density, the graphene-based anode showed a stable specific capacity of 225 mAh / g, whereas the specific capacity of the diatom-based anode was measured at 850 mAh / g. If the cathode capacity is fixed at 150 mAh / g, the total cell capacity can theoretically increase by more than 40% when using diatom-based anodes (see FIG. 16 b).

  Figures 17a and 17b illustrate the long cycle of the half cell (silica anode to Li counter electrode).

  These figures all show a battery (half cell having Li metal as a counter electrode) cycled 1000 times at a high charge / discharge rate. This indicates that the battery degrades at this C-rate, but in normal use the battery is not exposed to this high C-rate for a long time. A C rate of 500 mA / g is a very high discharge rate. In most types of electronic applications where Li-ion batteries are used, a C-rate of 500 mA / g is never required. Batteries are known to degrade quickly when high discharge rates are used and to last long when the discharge rates are low.

  In FIG. 17a, the electrodes are diatoms coated with carbon and mixed with carbon black and Na-alginic acid binder (using water as solvent). In FIG. 17b, the electrode is diatomaceous earth coated with carbon and mixed with carbon black and Na-alginate binder (using water as solvent). Diatomaceous earth is another type of amorphous silica that is commercially available, but does not have the preferred form that diatoms have. This figure was included to show the benefits of utilizing the particular morphology provided by diatoms.

Explanation of terms:
The C rate refers to the discharge rate at the unit of mA / g.
OGDE (FIG. 17b) is an abbreviation for organic grade diatomaceous earth.
C1 indicates the first charge / discharge cycle, and C2-1001 indicates the next 1000 cycles. Thus, this means that the first cycle is performed at a charge / discharge rate (C rate) of 100 mA / g, and the subsequent 1000 cycles are performed at 500 mA / g.
0 to 2.5 V of diatom means that the diatom electrode was cycled between 0 V and 2.5 V. Similarly, OGDE was also cycled between 0 V and 2.5 V.

  FIG. 18 shows half cells cycled every other cycle at high and low discharge rates. In this figure, C rates (50 mA / g and 500 mA / g) are used alternately. This is closer to the use of the battery in practical applications. Therefore, here, the half cell containing diatoms is first charged and discharged at 20 mA / g in the first cycle. Thereafter, in the remaining cycles, the charge / discharge rates alternate between 50 mA / g and 500 mA / g. Thus, the upper row of squares represents charge and discharge at 20 mA / g, respectively. The lower series of squares represent charge and discharge at 500 mA / g, respectively. The electrodes here are diatoms similar to those described above, coated with carbon and mixed with carbon black and Na-alginic acid binder using water as solvent. The electrodes were cycled between 0.015V and 2.5V.

  In FIG. 19, the electrode is a diatom similar to that described above, coated with carbon and mixed with carbon black and Na-alginate using water as the solvent. Here, the half cell was cycled with a rather narrow potential window of 0.02V to 2.5V. The first cycle is performed at 20 mA / g and the next cycle is performed at 500 mA / g. In addition, 10% FEC (fluoroethylene carbonate) was added to the electrolyte. This is a common additive in battery electrolytes and promotes stability. This is also apparent as the degradation observed when not using FEC (ie, FIG. 17a) is not seen.

FIG. 20 shows charge / discharge data for two half-cells, one cycled using CNTs as the conductive additive in the binder and one cycled using a Na-alginic acid binder without CNTs. CNT-containing cells: carbon-coated diatoms dissolved in water, mixed with carbon black and Na-alginic acid binder containing CNTs.
CNT-free cells are: carbon-coated diatoms dissolved in water, mixed with carbon black and Na-alginic acid binder. The upper two lines are the charge (blue) and the discharge (red) of cells containing CNTs in the binder. The lower two lines are for cells containing only Na-alginate binder. This represents an increase over about 250 mAh / g to 300 mAh / g when carbon black is replaced with CNTs. Each cell is cycled at 100 mA / g.

  A Nyquist diagram of both cells is shown in FIG. The Nyquist plot shows cell impedance data showing much lower resistance for cells containing CNTs. Blue lines indicate cells containing CNTs. The values on the x-axis indicate resistance values in ohms. By stretching the blue and red semicircles along the x-axis, it can be seen that the resistance of cells containing CNTs is slightly below 40 ohms, while cells without CNTs in the binder have resistances exceeding 60 ohms. Low internal resistance improves capacity at both high and low C rates.

The results of the all cell test are shown in FIG. This figure shows the results of an all-cell battery using commercially available NMC [Li (Ni, Mn, Co) O ₂ ] as the cathode and diatoms containing CNTs added to the binder as the anode. This data indicates that diatoms actually function in the whole cell form. The blue line shows the first charge and discharge of the cell and the red line shows the second charge and discharge cycle. This cell cycles between 2.5V and 4.2V. The composition of the anode is: carbon-coated diatoms, carbon black, Na-alginic acid binder including CNT, water cathode as solvent, using carbon black as conductive additive, PVDF as binder, NMP as solvent, It is made according to the standards of

<Conclusion>
The extension of the cycle, the reversible capacity of the carbon coating diatoms (SiO ₂ -C) begins to increase from the fourth cycle per a chemical reaction leading to Si phase. The porous morphology and natural nanostructures of carbon-coated diatom-based anodes can accommodate the volume expansion associated with Si and protect the solid electrolyte interface layer. The specific discharge capacity of these diatom-based anodes is more than twice the value of the existing theoretical capacity of graphite-based anodes. The high reversible capacity, good cycling performance and simple processing of these electrodes make diatoms a potential environmentally friendly high capacity anode material for lithium ion batteries.

Claims (19)

A porous silicon dioxide network coated with a carbon coating,
Conductive fillers such as carbon black,
A composite comprising a water dispersible or water soluble binder, preferably an alginic acid binder. A calcined diatomaceous silicon dioxide network coated with a carbon coating,
Conductive fillers such as carbon black,
A composite comprising a water dispersible or water soluble binder, preferably an alginic acid binder.   A composite according to any of the preceding claims, wherein the filler is carbon black.   A composite according to any of the preceding claims, wherein the binder is an alginate, such as sodium alginate.   A composite according to any of the preceding claims, wherein the binder comprises CNTs.   A composite according to any of the preceding claims, wherein the network comprises less than 1% by weight KCl and / or NaCl.   A composite according to any of the preceding claims comprising 10 to 40 wt% carbon coating in the silicon dioxide network, based on the weight of the coated silicon dioxide network.   A composite according to any of the preceding claims comprising 15 to 50% by weight of filler.   A composite according to any of the preceding claims comprising 5 to 30% by weight of a binder. 30-80% by weight of silicon dioxide network,
15 to 50% by weight of filler,
A composite according to any of the preceding claims comprising 2.5-30% by weight of a binder.   A composite according to any of the preceding claims, wherein the carbon coating coated silicon dioxide network is 30-80% by weight.   A porous silicon dioxide network coated with a carbon coating, the porous silicon dioxide network coated with a carbon coating comprising KCl and NaCl in a total weight of less than 1% by weight.   An anode for a lithium ion battery, comprising the composite according to claim 1.   A lithium ion battery comprising at least an anode, a cathode and an electrolyte, the anode comprising the composite according to claims 1-11.   15. The lithium ion battery according to claim 14, having a capacity of at least 700 mAh / g after 50 cycles of charge and discharge. Calcining a diatom source in the presence of a carbon source to obtain a carbon-coated calcined diatom network;
Mixing the network with a conductive filler, such as carbon black, and a water soluble / water dispersible binder. (I) procuring a diatom source, for example, from the ocean, and optionally drying the diatom;
(Ii) washing the diatoms and optionally drying the washed diatoms;
(Iii) calcining the product of step (ii) under an inert atmosphere at a temperature of 400-800 ° C., a method of making a porous carbon-coated silicon dioxide network. (I) procuring a diatom source, for example, from the ocean, and optionally drying the diatom;
(Ii) purifying the diatoms and optionally drying the purified diatoms;
(Iii) Baking the diatom of step (ii) at a temperature of 400 to 800 ° C. in an atmosphere containing oxygen,
(Iv) adding a carbon source to the baked diatoms of step (iii);
(V) calcining the product of step (iv) at a temperature of 400-800 ° C. under an inert atmosphere, a method of making a porous carbon-coated silicon dioxide network.   A binder suitable for use in LIBs comprising alginate and carbon nanotubes.